Elements of geology, or, The Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments elementsofgeolog00lyel Year: 1868 Ch. XXXVI.] METAMORPHIC ROCKS. W are four distinct forms of structure exhibited in rocks, namely, strati- fication, joints, slaty cleavage, and foliation ; and all these must have different names, even though there be cases where it is impossible, after carefully studying the appearances, to decide upon the class to which they belong. Professor Sedgwick, whose essay ' On the Structure of large Min- eral Masses ' fir
Elements of geology, or, The Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments elementsofgeolog00lyel Year: 1868 Ch. XXXVI.] METAMORPHIC ROCKS. W are four distinct forms of structure exhibited in rocks, namely, strati- fication, joints, slaty cleavage, and foliation ; and all these must have different names, even though there be cases where it is impossible, after carefully studying the appearances, to decide upon the class to which they belong. Professor Sedgwick, whose essay ' On the Structure of large Min- eral Masses ' first cleared the way towards a better understanding of this difficult subject, observes, that joints are distinguishable from lines of slaty cleavage in this, that the rock intervening between two joints has no tendency to cleave in a direction parallel to the planes of the joints, whereas a rock is capable of indefinite subdivision in the direction of its slaty cleavage. In cases where the strata are curved, the planes of cleavage are still perfectly parallel. This has been observed in the slate rocks of part of 'Wales (see fig. 75S), Parallel planes of cleavage intersecting curved strata, (Sedgwick.) which consist of a hard greenish slate. The true bedding is there indicated by a number of parallel stripes, some of a lighter and some of a darker color than the general mass. Such stripes are found to be parallel to the true planes of stratification, wherever these are manifested by ripple-mark, or by beds containing peculiar organic remains. Some of the contorted strata are of a coarse mechanical structure, alternating with fine-grained crystalline chloritic slates, in which case the same slaty cleavage extends through the coarser and finer beds, though it is brought out in greater perfection in propor- tion as the materials of the rock are fine and homogeneous. It is only when these are very coarse that the cleavage planes entirely van- ish. These planes are usually inclined
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